As high-density mounting of semiconductor devices is promoted, bumps made of solder or the like have been heretofore used for connecting semiconductor chips to substrates, and in order to directly connect them, bumps in the form of balls each having a diameter of about 100 μm have been used frequently. If a back surface of a wafer provided with such high bumps on a circuit surface thereof is ground, a difference in pressure due to a difference in bump height exerts influence on the back surface directly, and this influence cannot be absorbed by the cushioning effect of an adhesive sheet that is provided for surface protection. As a result, the wafer is broken during the grinding process, or a dimple (concave produced on the back surface) is formed to deteriorate reliability of the completed device. In order to avoid such problems in the conventional technique, the finished thickness is made relatively large so that breakage of wafer should not occur or designing is made so that bumps should be arranged in a low density.
In recent years, however, arrangement of high bumps in a high density has been required for many devices. If a usual adhesive sheet “A” for surface protection is used for such devices, the adhesive layer cannot be stuck to the edge of the wafer because of an obstacle of the bump, as shown in FIG. 4. As a result, a part of cleaning water that is sprayed for the purpose of removing heat or grinding dust generated during the back surface grinding process sometimes penetrates to the circuit surface side, thereby fouling the circuit surface.
To cope with the above problem, the thickness of the adhesive layer is increased and the fluidity of the adhesive is increased to bring the adhesive layer and the edge of the wafer into close contact with each other. However, because the adhesive is liable to come round onto the root portions of the bumps, the adhesive having adhered to the root portions of the bumps undergoes layer internal rupture by the peeling operation of the adhesive sheet, and a part of the adhesive is left. This problem may occur also in the case where an adhesive sheet using an energy ray-curable type adhesive is used. If the adhesive left on the circuit surface is not removed by solvent cleaning or the like, it remains as a foreign substance in the completed device and impairs reliability of the device.
Japanese Patent Laid-Open Publication No. 62950/1993(“JP '950”), there is disclosed a method for sticking a protective tape to a semiconductor wafer, wherein a protective tape capable of controlling adhesive strength by proper treatment is used and the protective tape is stuck only to the peripheral region of a semiconductor wafer with a strong adhesive strength. To be brief, in this method, an ultraviolet ray-curable type adhesive tape is used as the protective film, and prior to sticking of the tape to the semiconductor wafer, the adhesive layer which is to be brought into contact with the circuit-formed region of the wafer is cured, and fixing of the wafer is carried out only in the peripheral region of the wafer.
In the method of the aforementioned JP '950, however, the cured adhesive layer and the uncured adhesive layer are present on the same plane. Therefore, if the height of the bump is increased, the adhesive layer cannot be stuck to the edge of the wafer because of an obstacle of the bump. On this account, such a defect that the cleaning water penetrates to the circuit surface side as shown in FIG. 4 has not been improved sufficiently.
The present invention has been made in view of such prior technique as mentioned above, and it is an object of the present invention to provide a surface protective sheet and a method for grinding a semiconductor wafer, by the use of which any dimple is not formed, nor occurs breakage and fouling of a wafer, even when a wafer having high bumps which are highly densely arranged is ground to an extremely small thickness, and besides, no adhesive is left at the roots of the bumps after the surface protective sheet is peeled.